1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a light-emitting device having an optical resonance layer that provides increased light coupling efficiency.
2. Description of the Related Art
Light-emitting efficiency of light-emitting devices, and in particular, flat panel displays such as liquid crystal displays (LCDs) and electroluminescent (EL) devices, is classified into internal efficiency and external efficiency. Internal efficiency depends upon photoelectric conversion efficiency of organic light-emitting materials. External efficiency, also referred to as light coupling efficiency, depends on refractive indices of the layers constituting an organic light-emitting diode (OLED). Since OLEDs have lower light coupling efficiency than other displays, such as cathode ray tubes (CRTs) or plasma display panels (PDPs), their display characteristics, such as brightness, lifetime, etc. must be improved.
The primary reason why OLEDs have lower light coupling efficiency than other displays is that when light is emitted from an organic layer in the OLEDs at an angle greater than a critical angle, total internal reflection occurs at an interface between a higher refractive-index layer, such as an ITO electrode layer, and a lower refractive-index layer, such as a substrate, thereby preventing the light from being emitted outside of the OLEDs. Thus, due to the total internal reflection at the interface, only about ¼ of the light emitted from the organic light-emitting layer may be emitted outside of the OLEDs.
Japanese Laid-Open Patent Publication No. Sho 63-172691 describes an OLED for preventing reduction of light coupling efficiency. The OLED includes a substrate capable of collecting light, such as a projection lens. However, such a projection lens cannot be easily formed on a substrate since pixels for the emission of the organic layer are very small.
Japanese Laid-Open Patent Publication No. Sho 62-172691 describes an OLED in which a first dielectric layer is interposed between a transparent electrode layer and a light-emitting layer, and a second dielectric layer is formed on the transparent electrode layer, the second dielectric layer having a refractive index corresponding to about an average of a refractive index of the first dielectric layer and a refractive index of the transparent electrode layer.
Japanese Laid-Open Patent Publication No. Hei 1-220394 describes an OLED of which a bottom electrode, an insulating layer, a light-emitting layer, and a top electrode are formed on a substrate and a mirror for reflecting light is formed on a sidewall of a light-emitting layer.
However, since the light-emitting layer is very thin, it is very difficult to install the mirror on the sidewall and thus, its installation may increase production costs.
To overcome these problems, Japanese Laid-Open Patent Publication No. Hei 11-283751 describes an organic light-emitting device having an organic layer or multi-organic layers interposed between an anode electrode and a cathode electrode and having diffraction lattices and a zone plate as constitutional elements. Light emitted from the organic layer can be extracted due to light scattering obtained by forming the diffraction lattices near an interface between layers having refractive indices different from each other. However, a production process of the diffraction lattice layer is complicated and since a surface of the diffraction lattice layer is curved, a thin layer formed on the diffraction layer cannot be easily patterned and a separate planarization process for filling curved portions of the surface is required.
Japanese Laid-Open Patent Publication Nos. Hei 8-250786, 8-213174, and 10-177896 describe OLEDs using a concept of optical microcavity.
The OLEDs have multi-layered translucent mirrors interposed between a glass substrate and an ITO electrode and the translucent mirrors together with a metal cathode electrode, which functions as a reflective layer, function as optical resonators. The translucent mirrors are formed by multi-layering a TiO2 layer having a high refractive index and a SiO2 layer having a low refractive index in turn. A reflectance can be controlled by varying the number of layer in the multi-layer and thus the optical cavity can be generated. As the number of layers constituting the translucent mirrors increases, the reflective property is improved. However, to control a reflectance of light having a specific wavelength, the number and thickness of the layers to be layered must be accurately chosen. Thus, the production process of an OLED is complicated. The OLEDs have high brightness and high color purity, but small view angle and narrow spectrum.
In full color displays, a thickness of a layer and a depth of cavity, etc., must vary according to red, green, and blue colors. Thus, the production process is complicated and the production costs are high.